Transcript ppt

Chapter 19: Carboxylic Acids
19.1: Carboxylic Acid Nomenclature (please read)
19.2: Structure and Bonding (please read)
19.3: Physical Properties. The carboxylic acid functional group
contains both a hydrogen bond donor (-OH) and a hydrogen
bond acceptor (C=O).
Carboxylic acids exist as hydrogen bonded dimers.
H-bond
acceptor
O
H-bond
C
H
donor
H3C
O
O
H O
H3C C
C CH3
O H
O
acetic acid
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19.4: Acidity of Carboxylic Acids. The pKa of carboxylic acids
typically ~ 5. They are significantly more acidic than water or
alcohols.
Bronsted Acidity (Ch. 1.13): Carboxylic acids transfer a proton
to water to give H3O+ and carboxylate anions, RCO2
O
R
C
Ka=
O
OH
+
H2O
R
[RCO2-] [H3O+]
[RCO2H]
typically ~ 10-5
for carboxylic acid
pKa
C
CH3CH3
~50-60
O
+
H3O
pKa= - log Ka
typically ~ 5 for
carboxylic acid
CH3CH2OH
16
PhOH
10
Increasing acidity
CH3CO2H
4.7
HCl
-7
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pKa ~16-18
H3C-H2C-OH
OH
+
H2O
R
+
H2O
+
+
O
pKa ~ 5
OH
+
H3O
pKa ~ 10
O
C
H3C-H2C-O
H3O
O
H2O
R
C
+
O
H3O
The greater acidity of carboxylic acids is attributed to
greater stabilization of carboxylate ion by:
a. Inductive effect of the C=O group
O 
C
R  O
b. Resonance stabilization of the carboxylate ion
O
O
C
H3C C
O
O
4 -electrons delocalized
over three p-prbitals
C-O bond length of a
carboxylates are the same
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19.5: Salts of Carboxylic Acids. Carboxylic acids react with
base to give carboxylate salts.
O
R
pKa
C
O
O
H
+
NaOH
R
5
C
Na
O
+
H2O
15.7
(stronger acid) (stronger base)
(weaker base) (weaker acid)
Detergents and Micelles: substances with polar (hydrophilic)
head groups and hydrophobic tail groups form aggregates in
Water with the carboxylate groups on the outside and nonpolar
tails on the inside
O
O
Steric acid
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19.6: Substituents and Acid Strength. Substituents on the
-carbon influence the pKa of carboxylic acids largely through
inductive effects. Electron-withdrawing groups increase the
acidity (lower pKa) and electron-donating groups decrease the
acidity (higher pKa). (see table 19.2, p. 800)
O
H
C
C
Cl
OH
H H
pKa
C
C
H3C
C
C
H3C
OH
C
C
C
OH
0.9
O
H3C
OH
H3C H
5.1
C
Cl Cl
O
H3C CH3
4.9
Cl
OH
1.3
O
OH
C
O
Cl H
2.9
H H
pKa
Cl
OH
H H
O
C
C
C
4.7
H3CH2C
O
O
C
C
H H
4.8
4.9
O
OH
H
C
C
OH
H H
4.7
Inductive effects work through -bonds, and the effect falls off
dramatically with distance
O
O
OH
Cl
Cl
OH
O
O
OH
OH
Cl
pKa
4.9
4.5
4.1
2.8
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19.7: Ionization of Substituted Benzoic Acids. The charge of
the carboxylate ion cannot be delocalize into the aromatic ring.
Electron-donating groups decrease the acidity. Electronwithdrawing groups increase the acidity. (Table 19.3, p. 802)
O
H
O
H3C
C
H
OH
C
O
OH
C
OH
H
pKa
4.7
4.3
O
4.2
O
OH
O
OH
R
OH
R
R
R= -CH3
-F
-Cl
-Br
-OCH3
-NO2
pKa
3.9
3.3
2.9
2.8
4.1
2.2
4.3
3.9
3.8
3.8
4.1
3.5
4.4
4.1
4.0
4.0
4.5
3.4
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19.8: Dicarboxylic Acids. one carboxyl group acts as an
electron-withdrawing group toward the other and lowers its pKa;
effect decreases with increasing separation
O
O
HO C (CH2)n C OH
pKa1
+ H2O
pKa2
O
O
O C (CH2)n C OH
+ H2O
O
O
O C (CH2)n C OH
+ H3O
Oxalic acid (n= 0) pKa1=
Malonic acid (n= 1)
Succinic acid (n=2)
Glutaric acid (n=3)
Adipic acid (n=4)
Pimelic acid (n=5)
+ H3O
1.2
2.8
4.2
4.3
4.4
4.7
pKa2= 4.2
5.7
5.6
5.7
5.4
5.6
19.9: Carbonic Acid (please read)
O C O
+ H2O
O
HO C OH
pKa1
+ H2O
O
O C OH
~ 6.4
+ H3O
pKa2
+ H2O
~ 10.2
O
O C O
+ H3O
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19.10: Sources of Carboxylic Acids. Summary of reaction from
previous chapters that yield carboxylic acids (Table 19.4, p. 805)
a. Side-chain oxidation of alkylbenzene to give benzoic acid
derivatives (Ch. 11.13): reagent: KMnO4
b. Oxidation of primary alcohols (Ch. 15.10)
reagent: H2CrO4/H2Cr2O7
c. Oxidation of aldehydes (Ch. 17.15)
reagent: H2CrO4/H2Cr2O7
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19.11: Synthesis of Carboxylic Acids by the Carboxylation of
Grignard Reagents. Conversion of an alkyl or aryl Grignard
reagent to a carboxylic acid with an addition carbon (the CO2H
group). The CO2H group is derived from CO2.
Mg(0)
R-Br
R-MgBr
CO2
O
R C O
MgBr
H3O
O
R C OH
Grignard reagents are strong bases and strong nucleophiles and
Are incompatible with acidic (alcoholc, thiols, amines, carboxlic
acid, amides,) or electrophilic (aldehydes, ketones, esters,
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nitrile, halides) groups.
19.12: Synthesis of Carboxylic Acids by the Preparation
and Hydrolysis of Nitriles. Cyanide ion is an excellent
nucleophile and will react with 1° and 2° alkyl halides and
tosylates to give nitriles. This reaction add one carbon. The nitrile
Can be hydrolyzed to a carboxylic acid
CN
R-Br
R-CN
O
R C OH
H3O
SN2
NaCN
H3CH2CH2CH2C-Br
DMSO
H3CH2CH2CH2C-CN
H3O+
+ NH4
H3CH2CH2CH2C-CO2H
C5
C4
Br
NaCN
C5
CN
H3O+
CO2H
DMSO
PhO
PhO
Cyanohydrins (Ch. 17.7) are hydrolyzed to -hydroxy-carboxylic
acids.
O
NaCN
HO CN
H3O+
HO CO2H
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19.13: Reactions of Carboxylic Acids: A Review and Preview.
a. Conversion to acid chlorides (Ch. 12.7). Reagent: SOCl2
R-CO2H
SOCl2

O
R C Cl
+ SO2 + HCl
b. Reduction to a 1° alcohol (Ch. 15.3). Reagent: LiAlH4
Carboxylic acids are reduced to 1° alcohols by LAH,
but not NaBH4.
R-CO2H
a. LiAlH4, THF
b. H3O+
RCH2OH
c. Acid-catalyzed esterification (Ch. 15.8)
Reagent: R’OH, H+ (-H2O)
R'OH, H+ (-H2O)
R-CO2H
O
R C OR'
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19.14: Mechanism of Acid-Catalyzed Esterification.
Fischer Esterification (Fig. 19.1, p. 809-810)
R'OH, H+
R-CO2H
O
R C OR'
+ H2O
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19.15: Intramolecular Ester Formation: Lactones. Lactones
are cyclic esters derived from the intramolecular esterification of
hydroxy-carboxylic acids. 4-Hydroxy and 5-hydroxy acids cyclize
readily to form 5- and 6-membered ring ( and ) lactones.
O
O
+
HO-CH2-CH2-CH2 C OH
H2O
O
-butyrolactone
O
O
O
HO-CH2-CH2-CH2CH2 C OH
+
H2O
-valerolactone
19.16: -Halogenation of Carboxylic Acids:
The Hell-Volhard-Zelinsky Reaction.
O
H
C
C
Br2, PBr3
OH
then H2O
O
Br
C
C
OH
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Mechanism of -halogenation goes through an acid bromide
intermediate. The acid bromide enolizes more readily than the
carboxylic acid. Mechanism is analogous to the -halogenation
of aldehydes and ketones
The -halo carboxylic acid can undergo substitution to give
-hydroxy and  -amino acids.
Br O
R C C OH
H
Br O
R C C OH
H
K2CO3, H2O

NH3, H2O
HO O
R C C OH
H
H2N O
R C C OH
H
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19.17: Decarboxylation of Malonic Acid and Related
Compounds. Carboxylic acids with a carbonyl or nitrile group
at the -position will decarboxylate (lose CO2) upon heating
H
O
O

C
C
HO  C
O
H H

OH
C
H
HO
C
H
+ CO2
HO
O
C
H
C
H H
malonic acid
R
O
C
H
O
C
C
H H

O
OH
C
H
R
C
H
+ CO2
R
O
C
H
C
H H
-keto-acid
Decarboxylation initially leads to an enol of the -carbonyl group.
This is a key step in the malonic acid synthesis (Ch. 21.8) and
the acetoacetic ester synthesis (Ch. 21.7).
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19.18: Spectroscopic Analysis of Carboxylic Acids
Infrared Spectroscopy
Carboxylic acids:
Very broad O-H absorption between 2500 - 3300 cm1
usually broader than that of an alcohol
Strong C=O absorption bond between 1700 - 1730 cm1
O-H
C=O
No
C=O
O-H
C-H
O
OH
OH
C-H
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NMR: The -CO2H proton is a broad singlet near  ~12. When
D2O is added to the sample the -CO2H proton is replaced by D
causing the resonance to disappear (same for alcohols). The
-CO2H proton is often not observed.
1H
13C
NMR: The chemical shift of the carbonyl carbon in the 13C
spectrum is in the range of ~165-185. This range is distinct from
the aldehyde and ketone range (~190 - 220)
-CO2H
(180 ppm)
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problem 19.34b
O-H
C=O
128.7
123.9
146.8
45.3
179.7
18.0
147.4
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